Patterns are of various types, each satisfying certain casting requirements.

Single piece pattern

Split or two piece pattern

Match plate pattern

Single Piece Pattern

The one piece or single pattern is the most inexpensive of all types of patterns. This type of pattern is used only in cases where the job is very simple and does not create any withdrawal problems. It is also used for application in very small-scale production or in prototype development. This type of pattern is expected to be entirely in the drag and one of the surface is is expected to be flat which is used as the parting plane. A gating system is made in the mold by cutting sand with the help of sand tools. If no such flat surface exists, the molding becomes complicated. A typical one-piece pattern is shown in Figure 6.

Figure 6: A Typical One Piece Pattern

Split or Two Piece Pattern

Split or two piece pattern is most widely used type of pattern for intricate castings. It is split along the parting surface, the position of which is determined by the shape of the casting. One half of the pattern is molded in drag and the other half in cope. The two halves of the pattern must be aligned properly by making use of the dowel pins, which are fitted, to the cope half of the pattern. These dowel pins match with the precisely made holes in the drag half of the pattern. A typical split pattern of a cast iron wheel Figure 7 (a) is shown in Figure 7 (b).

Figure 7 (a): The Details of a Cast Iron Wheel

Figure 7 (b): The Split Piece or Two Piece Pattern of a Cast Iron Wheel

Classification of casting Processes

Casting processes can be classified into following FOUR categories:

1. Conventional Molding Processes

Green Sand Molding

Dry Sand Molding

Flask less Molding

2. Chemical Sand Molding Processes

Shell Molding

Sodium Silicate Molding

No-Bake Molding

3. Permanent Mold Processes

Gravity Die casting

Low and High Pressure Die Casting

4. Special Casting Processes

Lost Wax

Ceramics Shell Molding

Evaporative Pattern Casting

Vacuum Sealed Molding

Centrifugal Casting

Green Sand Molding

Green sand is the most diversified molding method used in metal casting operations. The process utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture.

Advantages

Most metals can be cast by this method.

Pattern costs and material costs are relatively low.

No Limitation with respect to size of casting and type of metal or alloy used

Disadvantages

Surface Finish of the castings obtained by this process is not good and machining is often required to achieve the finished product.